Anaplasma phagocytophilum (Aph) is a tick-transmitted obligate intracellular bacterium of the family Anaplasmataceae that can infect humans, livestock, companion animals and wild animals. In addition to Aph, the Anaplasmataceae family members include Anaplasma marginale, Anaplasma platys, Ehrlichia chaffeensis, Ehrlichia canis, and Ehrlichia ruminatium, among others, and all of these cause similar infections known collectively as ehrlichiosis. When humans contract an Aph infection it is more specifically known as human granulocytic anaplasmosis (HGA). An emerging and potentially fatal disease, HGA is transmitted by the same vectors that transmit Lyme Disease, primarily ticks and deer, but other animal and human hosts can complete the vector cycle and therefore extend the spread of disease. Since HGA became a reportable disease in the U.S. in 1999, the number of cases has risen annually, reaching 1,761 in 2010. Since diagnostic tools for HGA are lacking, this number of actual cases is likely to be much higher. HGA is increasingly recognized in Europe and Asia, and Aph infection is now the most widespread tick-transmitted disease of animals in Europe.
As the name implies, an obligate intracellular bacterium must enter a target cell in its human or animal host to survive, replicate, and move to the next host. When an Anaplasma spp. or Ehrlichia spp. infected tick bites a subject, the bacteria is transferred from the tick salivary glands into the tissues of the host or into the bloodstream where they bind to the surface of host cells and are taken up into vacuoles that form around each bacterium. A resident bacterium prevents the vacuole from merging with lysosomes. In doing so, the bacterium converts the cell into a protective niche that favors bacterial survival and remains in circulation to enable it to complete its zoonotic cycle. While the hallmark of these diseases is Aph colonization of neutrophils, other pathological findings can include leukopenia, thrombocytopenia, and elevated serum transaminase levels. Anaplasmosis is marked by fever and increased susceptibility to potentially fatal opportunistic infections.
Infected neutrophils in a host can be ingested in a second tick bite/bloodmeal. In this manner the disease may be transferred to another subject bitten subsequently. However, HGA can also be transmitted perinatally or by blood transfusion, and possibly nosocomially. Blocking infection of neutrophils could conceivably prevent all of these types of dissemination of Aph infection and the increased risk of opportunistic infections that can accompany the disease. Furthermore, targeting Aph proteins that are conserved among related Anaplasmataceae family members might also reduce or block transmission of disease caused by related Anaplasmataceae family members.
In addition to neutrophils, Aph has been detected in the microvascular endothelium of heart and liver in experimentally infected severe combined immunodeficiency mice. Promyelocytic and endothelial cell lines are useful in vitro models for studying Aph-host cell interactions. When naïve neutrophils or HL-60 cells are overlaid on Aph-infected endothelial cells, the bacterium rapidly transmigrates into the myeloid cells. These observations demonstrated that Aph infects endothelial cells in vivo and suggested that the bacterium may transmigrate between endothelial cells and neutrophils during the course of mammalian infection. Thus, targeting the Aph invasins that mediate infection of endothelial cells may prevent movement of the bacteria into the microvasculature of heart and liver in an affected subject by blocking the mechanism of endothelial cell-to-neutrophil transfer.
The Aph genome has now been sequenced and annotated. The annotated Aph genome (HZ strain, isolated from a human patient) can be found on the website at cmr.jcvi.org, and more specifically on the page found at cmr.jcvi.org/tigr-scripts/CMR/GenomePage.cgi?org_search=&org=gaph. Another website is the page found at ncbi.nlm.nih.gov, more specifically on the page found at ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?mode-Info&id=948&1v1=3&keep=1&srchmode=1&unlock. This information has promoted some progress regarding diagnosis of anaplasmosis and protecting subjects from infection. U.S. Pat. No. 7,906,296 B2 to Beall et al teaches that Anaplasma platys (Apl), formerly known as Ehrlichia platys, causes tick-born anaplasmosis in dogs. Beall further teaches the uses of Apl polypeptides. The Apl sequences were amplified from a blood sample of a dog known to be infected with Apl in order to develop methods for detecting the presence of antibodies to Apl and/or Aph in dog sera. Beall also teaches that peptides including Apl P44, encoding a major surface protein, might be used to elicit an immune response in vivo and confer resistance to anaplasmosis caused by Apl or Aph. U.S. Pat. No. 8,158,370 B2 to Liu el al. teaches that polypeptide sequences amplified from either Apl or Aph can be used in diagnostic assays, or to induce an immune reaction that might confer protection from Apl or Aph infection. The Aph sequences used by Liu were derived from APH_0915, encoding a protein of unknown function. More recently, Liu et al., in U.S. Pat. No. 8,303,959 and US 2013/0064842, teaches the use of four strain variants of Aph P44 surface proteins to diagnosis and protect against anaplasmosis.
Despite these teachings, there are currently no human or animal vaccines against anaplasmosis caused by any of the Anaplasmataceae family members. Antibiotic treatments with doxycycline or tetracycline can be effective, but tools for rapid and definitive diagnosis in any species other than dogs are lacking. The current gold standard serologic test for diagnosis of anaplasmosis in humans is indirect immunofluorescence assays (IFA) performed as timed pairs over a period of a few weeks, only available in specialized reference laboratories. This assay measures non-specific increases in IgM and IgG antibody levels. However, IgM antibodies, which usually rise at the same time as IgG near the end of the first week of illness and remain elevated for months or longer, are even less specific than IgG antibodies and more likely to result in a false positive. Serologic tests based on enzyme immunoassay (EIA) technology are available from some commercial laboratories. However, EIA tests are qualitative rather than quantitative, meaning they only provide a positive/negative result, and are less useful to measure changes in antibody titers between paired specimens. Furthermore, some EIA assays rely on the evaluation of IgM antibody alone, which again may have a higher frequency of false positive results. Between 5-10% of currently healthy people in some areas may have elevated antibody titers due to past exposure to Aph or related family members. If only one sample is tested it can be difficult to interpret. A four-fold rise in antibody titer is needed to achieve significance in paired samples taken weeks apart.
Therefore, the need remains for compositions and methods to rapidly and accurately diagnosis new cases and to provide adequate vaccination against Anaplasmataceae infections that cause anaplasmosis and HGA.